J.-F. Leon

Measurement and modeling of the Saharan dust radiative impact: Overview of the Saharan Dust Experiment (SHADE)

[1] Aerosols are known to be important in determining Earth’s radiative balance. Dust aerosols are of particular interest since, in addition to their scattering and absorbing properties that affect the solar radiation, they also perturb the terrestrial radiation. Recent studies have shown that a significant proportion of mineral dust in the atmosphere may be of anthropogenic origin, and therefore they may have an important role in climate change by exerting a significant radiative forcing. However, the optical and radiative properties of dust are not yet very well-determined, and even the sign of the resulting forcing is still questionable. The Saharan Dust Experiment (SHADE) was designed to better determine the parameters that are relevant for computing the direct radiative effect. Two aircraft combining in situ and remote sensing instruments were coordinated with satellite overpasses and ground-based observations during the experiment, which was based in the Cape Verde area during the period 19–29 September 2000. These in situ and remotely sensed data provide new valuable information on the microphysical, optical properties, and radiative effects of a large mineral dust outbreak. In addition, a global chemical transport model was used for assessing the radiative impact of these events, which are shown to be important on regional and global scales. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 3359 Meteorology and Atmospheric Dynamics: Radiative processes; 3360 Meteorology and Atmospheric Dynamics: Remote sensing; KEYWORDS: Saharan dust, physical and optical properties, dust direct radiative forcing

Analysis of the spectral and angular response of the vegetated surface polarization for the purpose of aerosol remote sensing over land

A precise estimate of the polarization induced by the surface in reflected radiation is crucial for remote sensing applications dedicated to monitoring the atmosphere. Here we present airborne observations acquired during a field campaign in the North of France over vegetated surfaces. Polarized reflectances were measured in four spectral bands in the range between 0.67 and 2.2 μm and for scattering angles between 75° and 145°. Our results confirm that the polarization generated by the reflection of vegetated surfaces can be understood as being primarily a specular reflection process. It is not possible from our measurements to see any spectral dependence of the surface polarization in the given spectral channels. The surface polarization is well fitted by existing surface models which have two degrees of freedom that allow the magnitude and angular behavior of the surface-polarized reflectance to be adjusted.

Maritime and dust aerosol retrieval from polarized and multispectral active and passive sensors

The two-dimensional structure and the optical properties of Saharan dust and maritime aerosols have been investigated over the Mediterranean Sea during October 2001 using airborne and satellite observations. A new airborne downward looking multispectral (490–2200 nm) micropolarimeter (MICROPOL) is used to derive the aerosol optical thickness and effective radius. We present two case studies corresponding to the observations performed during a mineral dust transport from the Sahara to Europe and to an undisturbed marine boundary layer. The dust plume is associated with aerosol optical thickness above 0.2 (at 865 nm), whereas relatively low loading conditions are observed in the maritime case (down to 0.1). The MICROPOL-derived aerosol optical thickness is in an excellent agreement with coincident Moderate Resolution Imaging Spectroradiometer (MODIS) retrievals for both days. The effective radius retrieved by MICROPOL is also in a good agreement with the one from MODIS in the dust case. In the pure maritime case, this parameter is significantly underestimated by MICROPOL compared to MODIS retrieval. The vertical distribution of aerosol optical parameters is derived from combined two-wavelength backscattering airborne lidar observations and MICROPOL passive measurements. As expected in the case of a long-range transport, the aerosol effective radius within the dust layer is rather constant as a function of the altitude. A surprising low lidar depolarization factor of about 4% is retrieved within the dust plume, suggesting a major contribution of spherical particles. No significant depolarization has been observed in the marine boundary layer. For the given geometry of observation the retrieved aerosol models, which are based on the Mie theory, reproduce the MICROPOL polarized measurements within 5–8% in the dust and maritime case. The use of a nonspherical model increases by a factor of 2 the residual fitting error in polarization in the case of the dust observation. This result is confirmed by the lidar depolarization ratio and indicates that a large part of particles in the dust plume are spherical.

Simulation of aerosol radiative effects over West Africa during DABEX and AMMA SOP-0

The regional climate model RegCM3 has been used to assess optical properties and clear-sky direct radiative forcing (DRF) of mineral dust and carbonaceous aerosols over West Africa for the period October 2005 to April 2006. Our results display a significant seasonal variation of the aerosol single scattering albedo (SSA) due to varying contributions from biomass burning (BB) and dust. During December-January, simulated SSA values dropped to around 0.81-0.83 at 440 nm and to 0.80-0.85 at 675 nm when absorbing aerosols from biomass burning dominate the mixture. During March and April, when mineral dust dominates, simulated SSA values increased reaching around 0.90-0.92 at 440 nm and 0.94-0.96 at 675 nm. The simulated aerosol optical thickness (AOT) was maximum over central Africa where it far exceeded estimates of AOT from satellite which showed the greatest AOT in the gulf of Guinea. This discrepancy was linked to an overestimation of BB emissions in central Africa and a possible underestimation of AOT over central Africa due a high occurrence of cloud and associated difficulties in cloud screening. The DRF calculations were extremely sensitive to aerosol optical properties and underlying surface albedo. Over dark surfaces, the sum of shortwave (SW) and longwave (LW) top of the atmosphere (TOA) direct radiative forcing averaged from December to February was negative (−5.25 to −4.0 W/m2) while over bright surfaces it was close to zero (−0.15 W/m2). Large differences between SW surface and SW TOA direct radiative forcing indicated that SW absorption had an important influence on the radiative budget. The SW radiative heating rate associated with the aerosol reached 1.2 K/d at local noon (diurnal mean of 0.40 K/d) over Niamey (∼13.5°N, 2°E) and peaked at altitudes of 2-4 km, corresponding to the BB aerosol layer.

Multidirectional visible and shortwave infrared polarimeter for atmospheric aerosol and cloud observation : OSIRIS (Observing System Including PolaRisation in the Solar Infrared Spectrum)

The aim of this project is to improve the characterization of radiative and microphysical properties of aerosols and clouds in the atmosphere. These two atmospheric components and their interactions are among the main sources of uncertainty in the numerical forecast of climate change. In this context, we have designed a new airborne polarimeter for measuring directional, total and polarized radiances in the 440 to 2200 nm spectral range. This instrument is based on the POLDER concept, instrument that is currently aboard the PARASOL microsatellite. This new sensor consists in two optical systems for the visible to near infrared range (440 to 940 nm) and the shortwave infrared (940 to 2200 nm). Each optical system is composed of a wide field-of-view optics (114° and 105° respectively) associated to two rotating wheels for interferential filters and analysers respectively, and a 2D array of detectors. For each channel, the total and polarized radiances are computed using the measurements performed with the three analysers shifted by an angle of 60°. Thanks to the large field of view of the optics, any target is seen under several viewing angles during the aircraft motion. This type of instrument has been designed for the retrieval of optical thickness and microphysical properties of aerosols as well as for the determination of microphysical, macrophysical and radiative properties of clouds. In this paper, we will present this new instrument design and some preliminary results recently obtained during the first field campaign in May 2008 over Europe.

Extensive Comparison Between a Set of European Dust Regional Models and Observations in the Western Mediterranean for the Summer 2012 Pre-ChArMEx/TRAQA Campaign

The present analysis focuses on the model capability to properly simulate long-range Saharan dust transport for summer 2012 in the Western Mediterranean. The present contribution shows an intercomparison of a set of 9 European regional dust model simulations. An exhaustive comparison of model outputs against other models and observations can reveal weaknesses of individual models, provide an assessment of uncertainties in simulating the dust cycle and give additional information on sources for potential model improvement. The model outputs are compared against a variety of both ground-based and airborne in situ and remote sensing measurements performed during the pre-ChArMEx/TRAQA field campaign. For this kind of study, multiple and different observations are combined to deliver a detailed idea of the structure and evolution of the dust cloud and the state of the atmosphere at the different stages of the event

Variability of Mediterranean aerosols properties at three regional background sites in the western Mediterranean Basin

In the framework of the project ChArMEx (the Chemistry-Aerosol Mediterranean Experiment, http://charmex.lsce.ipsl.fr/), the variability of aerosol optical, microphysical and radiative properties is examined in three regional background sites on a southwest – northeast (SW–NE) straight line in the middle of the western Mediterranean Basin (WMB). The three sites are on the northward transport pathway of African dust:  Ersa, Corsica Island, France (43.00ºN, 9.36ºW, 80 m a.s.l),  Palma de Mallorca, Mallorca Island, Spain (39.55ºN, 2.62ºE, 10 m a.s.l) and  Alborán, Alboran Island, Spain (35.94ºN, 3.04ºW, 15 m a.s.l). AERONET (AErosol RObotic NETwork) sun-photometer products are mainly used. A preliminary analysis shows that at Ersa and Palma sites the annual aerosol optical depth (AOD) has a similar trend with a peak around 0.2 in July. The winter/spring AOD is lower in Palma than in Ersa, while it is reverse in summer/autumn. The aerosol particle size distribution (and the coarse mode fraction) shows clearly the SW–NE gradient with a decreasing coarse mode peak (and a decreasing coarse mode fraction from 0.5 - 0.35 - 0.2 in July) along the axis Alborán - Palma de Mallorca - Ersa. In addition to the seasonal and annual variability analysis, the analysis of AERONET products is completed with a large variety of ground-based and sounding balloons remote sensing and in situ instruments during the Special Observation Period (SOP) of the ADRIMED campaign in June 2013. The second part of the presentation will focus on the comparison of the observations at Palma de Mallorca and Ersa of the same long-range transported airmasses. The observations include lidar vertical profiles, balloon borne OPC (Optical Particle Counter) and MSG/SEVIRI AOD, among others.

Monitoring of ambient fine particulate matter concentrations from space: application to European and African cities

Air pollution is a major issue for global environment as well as human health and well-being. Recently, satellites which are equipped with relevant air quality instruments have been placed into orbit. In this paper, we first present a review on satellite remote sensing of particulate pollution. We then present new results for Europe and on African cities particulate air pollutants using POLDER satellite data. Based on satellite AOD observations, we show that the number of days exceeding the 15.4 μg/m3 threshold is twice frequent in Ouagadougou, Burkina- Faso than in Paris, France. At the regional scale, we observe that the northern coast of the golf of Guinea is dramatically impacted by poor air quality.

Interactions aérosols-rayonnement-climat en région méditerranéenne : Impact de l'effet radiatif direct sur le cycle de l'eau

An experimental campaign, coupled with three-dimensional modeling, was conducted in the western Mediterranean during the summer of 2013 to study the impact of aerosols on the radiative balance and climate of this region. In situ observations were obtained on the ground, aboard two research aircraft and balloons to characterize the physico-chemical and optical properties of particles and their vertical stratification. This campaign was mainly characterized by moderate events of desert aerosols. During these episodes, strong vertical stratification was observed and the measurements of the optical properties reveal moderate absorbing particles in the visible spectrum. Climate simulations indicate a significant impact of aerosols in particular by changing the surface temperature of the sea, the ocean-atmosphere fluxes and consequently seasonal precipitation.